In order to improve the viewing angle and avoid color changes in liquid-crystal display devices, retardation films that exhibit a specific retardation value (hereinafter, R value) have been used either singly or in combination.
Cellulose acetate is known to be advantageous as the main material of the retardation film and the optical characteristics of the film is known to depend on the degree of acetyl substitution of the cellulose acetate. In particular, cellulose acetate having a low degree of substitution has a high intrinsic birefringence, and therefore, by reducing the degree of acetyl substitution it is considered that the cellulose acetate can develop high optical characteristics suitable as a VA retardation film. As described in PTLs 1 to 4, a film of cellulose acetate can be directly laminated to a polarizer containing polyvinyl alcohol as a principal component by immersing the film in an alkaline aqueous solution to saponify and make the surface of the film hydrophilic. For this reason, the cellulose acetate films have been utilized as films that exhibit a retardation compensating function for a polarizer (hereinafter, retardation film).
A polarizer to which a retardation film is laminated is incorporated together with a liquid crystal cell when a liquid crystal display device is manufactured. Since the retardation film is disposed between the polarizer and the liquid crystal cell at this time, the optical characteristics of the film greatly affects the visibility of the liquid crystal display device. The retardation film is required to exhibit stable optical characteristics against various environmental changes.
For suitable development of optical characteristics, it is necessary to reduce the degree of acetyl substitution of a cellulose ester. However, when the degree of acetyl substitution of the cellulose ester is reduced, hydrogen bonding components increase in number due to the increased number of hydroxy groups thus resulting in a large variation in the development of optical characteristics. Particularly, this raises the following problem: “during film stretching optical characteristics are prone to vary in response to a change in the amount of residual solvent in the retardation film”. In recent years, with the wider viewing angle and higher image quality of the liquid crystal display device, high retardation compensation is increasingly required, and it has been required in the art to improve retardation compensation.
It has been proposed to use a film containing cellulose acetate having a high degree of acetyl substitution in order to stably develop optical characteristics even if the amount of residual solvent in the film changes when the film is stretched (for example, see PTL 5).